Turbine engines, such as single shaft industrial gas turbines, are designed to operate at a constant design turbine inlet temperature under any ambient air temperature (i.e., the compressor inlet temperature). This design turbine inlet temperature allows the engine to produce maximum possible power, known as base load. Any reduction from the maximum possible base load power is referred to as part load operation. In other words, part load entails all engine operation from 0% to 99.9% of base load power.
Part load operation may result in the production of high levels of carbon monoxide (CO) during combustion. One known method for reducing part load CO emissions is to bring the combustor exit temperature or the turbine inlet temperature near that of the base load design temperature. It should be noted that, for purposes of this disclosure, the terms combustor exit temperature and turbine inlet temperature are used interchangeably. In actuality, there can be from about 30 to about 80 degrees Fahrenheit difference between these two temperatures due to, among other things, cooling and leakage effects occurring at the transition/turbine junction. However, with respect to aspects of the present invention, this temperature difference is insubstantial.
To bring the combustor exit temperature closer to the base load design temperature, mass flow of air through a turbine engine can be restricted by closing the compressor inlet guide vanes (IGV), which act as a throttle at the inlet of the compressor. When the IGVs are closed, the trailing edges of the vanes rotate closer to the surface of an adjacent vane, thereby effectively reducing the available throat area. Reducing the throat area reduces the flow of air which the first row of rotating blades can draw into the compressor. Lower flow to the compressor leads to a lower compressor pressure ratio being established in the turbine section of the engine. Consequently, the compressor exit temperature decreases because the compressor does not input as much energy into the incoming air. Also, the mass flow of air through the turbine decreases, and the combustor exit temperature increases.
Some of the compressor exit air is used to cool structure of the turbine. This structure can include the outer casing, blade rings, and ring segments. In addition, some compressed air is piped directly out of the compressor through piping. This air is routed out of the engine, passed through a cooling circuit, and is ultimately redelivered to the engine at a substantially constant design cooling air temperature. The cooling circuit can include heat exchanger devices as well as valves for controlling the quantity of air passing through or bypassing the heat exchanger devices so as to achieve the design cooling air return temperature. The design temperature is held substantially constant so that the metal temperatures of the parts being cooled are held substantially constant, thereby maintaining the life of such parts. The design cooling return temperature can be specific to a particular engine design. This compressor bleed air is used to cool the stationery support structure near the second third and fourth rows of blades and is supplied through piping.
CO increases rapidly as gas turbine engine load is reduced below approximately 60%. Once IGVs have been closed to their limit, and the engine's exhaust temperature limit has been reached, then power can be reduced only by decreasing turbine inlet temperature (TIT). TIT reduction drops the combustion system's primary zone temperature (T_PZ), and CO and unburned hydrocarbons (UHC) are produced due to quenching of the combustion reactions in the turbine hot gas path. To prevent CO from increasing as engine load decreases, T_PZ must be maintained at a high level.